Plastic films have found utility in a wide variety of packaging applications such as for example bags, pouches, tubes and trays. In many film applications it is desirable to seal the film during the packaging operation. This may be accomplished by use of adhesives or by using heat sealing techniques. When heat sealing is used, it is important that the plastic film be readily heat sealable while also possessing other good physical and mechanical properties such as resistance to tearing, high tensile strength, and good processability in high speed equipment.
Film heat sealing is generally effected by means of heated flat surfaces, between which the films are forcefully pressed together at a temperature above the seal initiation temperature of the film. When use is made of equipment such as vertical form, fill and seal machines, the bag is filled with the contents to be packaged while the bottom seal is still hot. Cooling the seal would entail too long a waiting time, thus lengthening the cycle time and increasing operating costs. Consequently, the film must be one which enables the formation of a strong seal even as the seal formed is at or near the seal formation temperature.
It is evident that an important characteristic for a heat sealable film is the temperature at which the sealing begins, i.e. the heat seal initiation temperature. It is desired to operate at as low a temperature as possible because (1) it broadens the heat sealable range, (2) it permits higher productivity due to less time for cooling, (3) it requires less energy to heat seal at lower temperature, (4) at a lower heat seal initiation temperature, the film is more forgiving of inadequacies in the heat sealing equipment, and (5) the packaged food/product has less exposure to heat.
Many commonly used plastic materials that are used in the formation of film products could benefit from an improvement of their heat sealing characteristics. For example, crystalline polypropylene films have found extensive use in the field of packaging. Polypropylene films, in both oriented or non-oriented form, are used widely in packaging applications because of their superiority in mechanical properties such as tensile strength, rigidity, and surface hardness, optical properties such as gloss and transparency, and food hygiene such as freedom from toxicity and odor. However, polypropylene films, when coextruded with currently available sealant resins, typically require heat sealing initiation temperatures upwards of about 125° C., before adequate film seal strengths (at least 200 g/in) are obtained.
Considering the above, monolayer and coextruded films employing blends of propylene-based elastomers and polypropylene (PP) resins, and other polyolefin resins, overcome the heat seal limitations of the PP films mentioned above. Specifically, blends of propylene-based elastomers and PP resins, other polyolefin resins, and slip agents and antiblock agents produce monolayer and coextruded film with good mechanical properties, toughness, optical properties, and low coefficient of friction (COF). In addition, good hot tack and relatively low seal initiation temperatures can be obtained.